Theres a new, free game of VALVe called Alien Swarm, which is pretty much of a surprise as it was announced only three days before it was released, it is completely free and pretty good for that kinda game.It is from a top down perspective, you fight yourself your way through lots of aliens in a science fiction environment, and it requires quite some teamwork on higher difficulties.

Just played it with three friends together on Mumble (voice communication software like TeamSpeak 3), played till last map on hard difficulty, and only stopped because it was getting late. We had a good time there.

Woa, they just released a new update, it not only adds a higher difficulty level called Brutal, but also adds two options that you can toggle, one enabling some kind of AI Director like in Left 4 Dead, which spawns some random aliens in the levels and also random hordes, so the level is not the same every time, and another option to enable 100% friendlyfire, which forces you to take extreme care when shooting near teammates.

If you combine these three it gets insanely hard, we gotta try it somewhen, I gave up with a public team (all experienced players) after a lot of tries, because we didn't even get to the final of the first map. xD